Why does Kamdev exist in Sanata and Dharma in the first place?

Why not? That's such a powerful force. In fact, Dharma, Artha, Kamma, Moksa, there are the four goals that are put for life. So Kamma is already included among the objectives of life. It's not even something you don't look at it. Not like that. It's there.

So there's a deity. Why do Kamdev mantras exist in the first place?

There are mantras that exist to kill people also. Kamdev is a very good thing to have.

But what would that mantras effect be effectively?

Those mantras can be used in order for you have more opportunities for experiencing pleasure. Somebody wants to have an experience or something like that, I'm assuming, if those who have done Kamdev mantras.